Around 17,000 litres of whey has spilled into the Pangaki Stream near Piopio following a road accident this morning.

A truck and trailer unit carrying whey overturned on State Highway 3 at around 10 am. Environment Waikato estimates around 17,000 litres of whey has drained into the Pangaki Steam which in turn flows into the Mangaotaki River. The Mangaotaki River is a tributary of the Mokau River.

Environment Waikato Resource Officer Graeme Cooper said the whey is not toxic, but will affect fish life.

"The whey 'sucks' oxygen out of the water, so we are expecting fish to be adversely affected. At this stage, we don't know how badly," he said.

Mr Cooper said anyone who was taking water out of either the Pangaki Stream, Mangaotaki River or Mokau River south of the spill should stop until at least Thursday evening.

No resource consents to take water from the affected areas have been issued. However, people are allowed to take water for domestic and stock purposes without advising the Regional Council.

Mr Cooper said Environment Waikato expected the whey to be discharged to the coast in around 24 hours. Council officers would continue to monitor the situation for the next 48 hours, he said.